1. Field of the Invention
This invention relates to a brake fluid pressure control apparatus in a skid control system for a vehicle having at least one wheel and a brake for the wheel in which the brake fluid pressure to the wheel cylinder of the brake for the wheel is controlled in accordance with the rotational condition or skid condition of the wheel, and more particularly to a brake fluid pressure control apparatus of the type in which, when the brake for the wheel is relieved, brake fluid is discharged through a fluid pressure control valve device from the wheel cylinder of the brake into a hydraulic reservoir.
2. Description of the Prior Art
Japanese Patent Opening No. 142733/1981 (U.S. Pat. No. 4,395,073) discloses a brake fluid pressure control apparatus in a skid control system for a vehicle having at least one wheel and a brake for the wheel which includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel The fluid pressure control valve device receives control signals from a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder. A hydraulic reservoir, reserves the brake fluid discharged through said fluid pressure control valve device from said wheel cylinder when the brake fluid pressure to said wheel cylinder is decreased with control of said fluid pressure control valve device. A pressure fluid supply conduit connects the master cylinder with the fluid pressure control valve device and a fluid pump for returns the brake fluid from said hydraulic reservoir into said pressure fluid supply conduit. A first check valve is arranged in said pressure fluid supply conduit to open when the brake fluid flows from said master cylinder toward said fluid pressure control valve device. The outlet of said fluid pump is connected to said pressure fluid supply conduit between said first check valve and said fluid pressure control valve device. A pressure fluid return conduit connects said master cylinder with said wheel cylinder. A second check valve is arranged in said pressure fluid return conduit to open when the brake fluid flows from said wheel cylinder toward said master cylinder. A hydraulic accumulator receives the brake fluid discharged from said fluid pump. Indeed, kick-back of the brake pedal connected to the tandem master cylinder can be prevented with the above described brake fluid pressure control apparatus. However, the hydraulic accumulator includes a strong spring for accumulating a high pressure of fluid discharged from the outlet of the fluid pressure pump. Accordingly, the hydraulic accumulater is large in volume and is heavy. Further, the above brake fluid pressure control apparatus includes the first and second check valves and so the number of the parts is large. And since the brake fluid is temporarily accumulated in the hydraulic accumulator, the above brake fluid pressure control apparatus has the defect that the required amount of the fluid is increased.
Further, the Japanese patent opening No. 12452/1987 discloses the brake fluid pressure control apparatus in which a change-over valve apparatus is arranged between the master cylinder and the wheel cylinder and further, the electro magnetic cut-off valve is arranged between the wheel cylinder and the fluid pressure pump. The outlet of the fluid pressure pump is not directly connected to the master cylinder in the above arrangement. Accordingly, kickback of the pedal is very little. However, the pressurized brake fluid is discharged from the fluid pressure pump during the time when the electromagnetic cut-off valve is cut off. Accordingly, high pressure is applied to a seal portion of the electro magnetic cut-off valve. This may damage the seal portion of the electro magnetic cut-off valve. In that apparatus, when the pressure of the wheel cylinder is increased, the electro magnetic cut-off valve is put into the communicating state.